This invention relates to a method of treating metal surfaces to enhance corrosion resistant and paint bonding characteristics and more particularly, relates to trivalent chromium seal-coatings for anodized aluminum substrates.
It is generally known to treat the surfaces of metals, such as zinc, cadmium, or aluminum with aqueous chromate (hexavalent chromium) solutions which contain chemicals that dissolve the surface of the metal and form insoluble films known as "chromate conversion coatings." These chromium coatings, are corrosion resistant and protect the metal from various elements which cause corrosion. In addition, it is known that chromate conversion coatings generally have good paint bonding characteristics and, therefore, provide an excellent base for paint or other finishes.
Although the aforementioned coatings enhance corrosion resistant and paint bonding properties, the coatings have a serious drawback, i.e., the toxic nature of the hexavalent chromium constituent. This is a serious problem from two viewpoints, one being the handling of the solution by operators and the other, the disposal of the used solution. The disposal problem, however, can be mitigated by reducing the hexavalent chromium to the comparatively innocuous trivalent form before disposal. This method is expensive and therefore can be a major cost factor in the overall metal treating process. Therefore, it is highly desirable to have coatings which are substantially free of hexavalent chromium, but at the same time capable of imparting corrosion resistant and paint bonding properties which are comparable to those imparted by conventional chromium coatings.
Of particular interest is the use of chromate conversion coatings on aircraft aluminum alloys due to the excellent corrosion resistance and the ability to serve as an effective base for paint. The baths used to develop these coatings contain chromates, i.e., hexavalent chromium, and it is the residual chromates in the coating that is largely responsible for the high degree of corrosion inhibition. However, these same chromates are highly toxic and their presence in waste water effluents is severely restricted. It would therefore, be desirable to provide a coating for aluminum and its alloys and, more particularly a seal coating for anodized aluminum utilizing relatively non-toxic chemicals that could serve as an alternative to the toxic hexavalent chromate coatings.
For example, in the prior art, trivalent chromium baths (U.S. Pat. No. 4,171,231) have been used to produce coatings on zinc or zinc plate to provide a decorative "clear to light blue finish" which is characterized as having superior corrosion resistance. These baths are stated to contain "trivalent" chromium as substantially the only chromium ion, with a fluoride ion, an acid other than nitric acid and an oxidizing agent. The operating range of the baths is at a pH ranging from about 2 to 4 and preferably between 1 and 3. The baths are used to achieve a single-dip chromate finish on all types of zinc plate. The implication is that the presence of the oxidizer, in situ, produces hexavalent chromium on the zinc surface without any oxidation or conversion of the trivalent chromium in the bath to the hexavalent form. Patentee discloses further that without the oxidizing agent in the bath, corrosion resistance on the zinc plate was poor, i.e., extensive corrosion after 24 hours with a 5% salt spray exposure, whereas with the oxidizing agent in the bath there was 0-10% of white corrosion and some panels were free of white salt after 50 hours of salt spray exposure.
This invention, in comparison, utilizes trivalent chromium as the only chromium ion in the solution at a specific pH range and is specifically applied to aluminum alloys. It was found that the addition of an oxidizing agent such as peroxide to the solution, in situ, slowly oxidized the trivalent chromium to the toxic hexavalent form. This conversion to the hexavalent form is contrary to the method used by this invention; namely, utilization of a bath composition completely free of hexavalent chromium and containing no oxidizing agent.
With regard to anodized aluminum it is known to apply anodic coatings to aluminum by making the metal anodic in a suitable solution and with a suitable counter electrode (cathode). The application of an anodic current converts the surface of aluminum to aluminum oxide which is characteristically hard and wear resistant. The anodic coatings are usually microporous and can be sealed with dyes to obtain desired colors or with other solutions to improve corrosion resistance or to attain desired surface characteristics.
Some of the more commonly used solutions for applying anodic coatings on aluminum include sulfuric acid, chromic acid, oxalic acid, sulfophthalic acid, boric acid or combinations of these. For example, aluminum alloys are readily anodized in 15 percent sulfuric acid by application of 12 amps/ft.sup.2 anodic current at 21.degree. C. The anodic coatings usually range from 0.1 to 1 mil in thickness depending upon treatment time and alloy. The anodic coatings can be sealed in boiling water which hydrates and swells the coating and thereby closes the pores. In this manner, the corrosion resistance is improved and the surface less susceptible to staining by contact with colored materials. However, for optimum corrosion resistance, normally required for military equipment, anodic coatings are sealed in dichromates, usually 5 to 30 minute immersion in 50 g/l sodium dichromate solution at 85.degree. to 100.degree. C. The dichromate absorbed by the coating acts as an effective inhibitor to provide long lasting corrosion resistance even in very severe environments. For example, a proprietary anodic hard coating applied to 7075-T6 aluminum M-16 rifle components had a strong tendency to undergo catastrophic exfoliation corrosion during service in Vietnam. Sealing in dichromate prevented this corrosion phenomenon.
The dichromate (hexavalent chromium) has a serious drawback, however, inasmuch as the compound is highly toxic and presents an environmental health hazard to operators. Moreover, the presence of hexavalent chromium in waste effluents are severely restricted. In order to dispose of hexavalent chromium, it is necessary to use a chemical reducing agent to convert the hexavalent to the much less toxic trivalent form. The reduction process is expensive and reduction is often incomplete which leads to violations of EPA standards. Therefore, it is highly desirable to achieve corrosion resistance of anodized aluminum comparable to that of dichromate sealing, by utilizing relatively nontoxic chemicals that can serve as an alternative to the use of hexavalent chromium. Accordingly, this invention utilized trivalent chromium as the only chromium ion in the bath for the treatment or sealing of anodized aluminum.